a) Field of the Disclosure
The disclosed subject matter relates to apparatus and methods for harnessing energy from a pressure differential in a fluid flow stream, and in particular to the production of mechanical output and/or the generation of electrical power therefrom.
b) Background Art
It has been attempted to harness the potential energy in a flowing fluid by conventional reciprocating piston, turbine, or other means in order to drive a motor, or to otherwise generate mechanical or electrical power. The existence of a reliable source of fluid flow and pressure such as may for example be provided by tidal flows, wave energy, or at gas well sites, provides an attractive renewable energy source for the generation and supply of electrical power. This is particularly at remote locations where no line power is available, and where other energy sources such as solar or wind power are not viable.
Numerous gas-actuated reciprocal and turbine devices have been proposed for utilization of the energy of a pressure drop in a fluid flow stream to perform some form of work. Those that are intended for use in gas well applications also typically avoid venting the gas to the atmosphere, which is particularly important when the drive gas is explosive or corrosive (e.g. sour gas).
By way of example, Canadian patent no. 2,491,298 to Paval provides a closed-loop, gas-actuated reciprocal drive apparatus that utilizes the potential energy in the pressure differential existing between areas of higher and lower pressure in a natural gas pipeline to actuate the apparatus without exhausting the gas to the atmosphere. The reciprocating motion of the apparatus is in turn used to drive a compressor or chemical pump.
U.S. Pat. No. 6,336,389 to English describes an apparatus wherein a similar reciprocating motion is generated, but is then transferred through a linkage to produce rotational motion, which can subsequently be used to drive an injection pump or similar type of equipment. U.S. Pat. No. 6,694,858 to Grimes also describes an apparatus that produces a reciprocating motion for driving reciprocating equipment without expelling the gas to the atmosphere, and instead directing it back into the pipeline.
U.S. patent publication no. 2010/0133834 to Nimberger provides a differential pressure generator in which a bladed turbine rotor (rather than a reciprocating piston) is actuated by a controlled stream of fluid, and in which the electrical circuitry of the generator is isolated from the fluid stream without the use of dynamic seals in order to enhance safety.
While gas-actuated reciprocal and turbine devices have utility in some applications, their efficiency may be limited by frictional and/or inertial losses (particularly in the case of reciprocating devices), and/or by blow-by of the drive gas in the case of turbine devices. As a result, such devices may not be suitable for low-flow well sites or the like. In an effort to overcome these limitations, positive displacement expanders in which, for example, a Wankel rotary engine is driven by the differential fluid pressure (instead of a reciprocal or turbine apparatus) have also been utilized.
One example of a positive displacement device in which energy from a pressurized gas is converted to mechanical energy is provided in U.S. patent publication no. 2010/0215531 to Arnold. This publication describes a compressed air motor comprising a gear pump in which an intermeshing shaft rotor and a counterpart rotor are driven by the compressed gas to effect rotation of a power takeoff shaft. The speed ratio of the shaft and counterpart rotors of the Arnold motor may generally be expressed by the equation (1+1/N):1, such that one of the two rotors will have one additional lobe or “tooth” as compared to the other. To accommodate this speed ratio, the rotors of the Arnold motor (and other similar positive displacement devices) are formed with a “wave” or smooth gear-like profile that exhibits a pseudo rolling action from lobe-to-lobe during operation. Although this helps to ensure that gas cannot flow through the motor without driving the rotors, it also makes the rotors susceptible to grinding and jamming by particles of debris that may be present in the pressurized gas stream.
There accordingly exists a need for devices that are able to efficiently harness energy from a pressure differential in a fluid flow stream, and that in some variations are suitable for use in dirty environments (such as natural gas wells) in which impurities or debris may be found in the fluid flow stream.